<!DOCTYPE html>
<html>
	<head>
		<meta charset="UTF-8">
		<title></title>
	</head>
	<body>
	</body>
	<script type="text/javascript">
		var obj={
			a:undefined
		}
		
		console.log("a" in obj);//true
		console.log("b" in obj)//false
	</script>
</html>
